I can't tell you how much maple extract to use. However I can suggest that anyone wanting to brew a maple beer not limit themselves to actual maple, but to cheat by using an Indian spice known as fenugreek. Buy some, smell it, and imagine Mrs. Butterworth. Yes, I'm about 99.9% certain that fenugreek is used in all big commercial syrups, and is the defining ingredient of what people think of when they hear the word "maple", even though it has nothing to do with maple at all. Kind of like how witbier tastes like orange citrus from the coriander and not the orange peel (typically), or how LP gas smells skunky, it is because of the artificial additives that give us these ideas even moreso than the natural ingredients in them.
"This is grain, which any fool can eat, but for which the Lord intended a more divine means of consumption. Let us give praise to our Maker, and glory to His bounty, by learning about... BEER!" - Friar Tuck (Robin Hood - Prince of Thieves)